A tunable wavelength filter has a function for selecting a predetermined wavelength light signal from wavelength multiplexing light signals, and is one of key devices which is applied to a wide use such as an optical transmission system, an optical switching, an optical information processing system and so on. In any of the uses, the tunable wavelength filter is required to have a satisfactory wavelength selectivity and a wide wavelength tuning range. Furthermore, the tunable wavelength filter is desired to be a transmission type wavelength selection filter because it must be fabricated in the form of an optical integrated circuit.
Some transmission type wavelength selection filters are studied in the past. Among them, a semiconductor wavelength tunable filter is expected to be put into a practical use because it is suitable for an optical integrated circuit. In this point, it has been proposed that a distributed feedback semiconductor laser (DFB LD) is biased below an oscillation threshold level, thereby being used as a tunable wavelength filter. It is reported that theoretical high gain and narrow transmission band width are obtained in such a wavelength tunable filter. One of such reports is described on pages 123 to 126 of "Optics Communications, February 1974, Volume 10, number 2". Furthermore, device characteristics are described on pages 118 to 121 of "Technical Digest of 1st Microoptics Conference" of "1st Microoptics Conference MOC '87 Tokyo" held on October 15 and 16 at Ishigaki Memorial Hall, Tokyo, Japan.
According to the tunable wavelength filter in which the DFB LD is biased below an oscillation threshold level, however, there is a disadvantage that an optical gain and a spontaneous emitting light intensity are varied in regard to a selected wavelength, although the selected wavelength is tuned by controlling the carrier density injected into an active layer thereof. As a result, the carrier density injected into the active layer is limited in principle to be narrow for providing an appropriate wavelength selectivity in an optical filter and a predetermined intensity ratio of light in regard to noise. Consequently, the tuning range of selection wavelength is narrow, and the number of channels is only several.